Section 508
All web pages should strive to be Section 508 compliant.
Checklist
WebAIM has an excellent Section 508 checklist of things to pay attention to while working on web pages.
Web Content Accessibility Guidelines (WCAG) 2.0
Our ultimate goal is to be compliant with WCAG 2.0.
Testing
To check compliance of specific web page, you can test with WAVE.